In a typical wireless communications system, a base station communicates with a plurality of mobile stations over a wireless electromagnetic link operating in an allocated frequency range. In the case of a code-division multiple-access (CDMA) spread-spectrum system, such as that defined by ANSI standard J-STD-008 or TIA/EIA standard IS-95A, which are incorporated by reference herein, the allocated frequency range is divided into CDMA channels of width 1.23 MHz. Each CDMA channel is further split up into 64 code channels sharing the entire 1.23 MHz bandwidth of the CDMA channel, each code channel being associated with a respective one of a set of 64 mutually orthogonal spreading codes, hence the term "code division".
Each mobile station in the system is assigned one of the 64 code channels, along with a CDMA channel which it may share with other mobile stations having been assigned this same 1.23 MHz CDMA channel but other code channels. Therefore, in theory, up to 64 mobile stations can share the same CDMA channel. In practice, however, several reserved code channels may be occupied by a pilot channel, a sync channel and up to seven paging channels, allowing at least 55 (and up to 61) mobile stations to share the same 1.23 MHz CDMA channel. Incidentally, a code channel occupied by a mobile station is known as a "traffic" channel in the IS-95A specification and as a "fundamental" channel in the IS-95B specification.
In the forward (base-station-to-mobile-station) direction, a base station "spreads" the data destined for a particular mobile station by, e.g., multiplying it with the spreading code associated with the code channel assigned to the mobile station in question. Usually, the spreading code is a specially selected bit sequence having a rate that is much higher than the rate of the data. The spread data destined for many different mobile stations is combined, converted to analog form, modulated about a high-frequency carrier (to bring it within the frequency range of the appropriate CDMA channel) and finally amplified by a power amplifier prior to transmission by an antenna at the base station.
At each mobile station, the received signal in the assigned CDMA channel contains the spread data destined for it and for up to 54 or even 60 other mobile stations. However, because the spreading codes are mutually orthogonal, it becomes straightforward for the mobile station to isolate the code channel carrying the data spread by a single spreading code, and hence to recover the data contained therein.
Naturally, it is important to transmit the signal in any given 1.23 MHz CDMA channel at a sufficiently high power and with sufficient fidelity to enable good reception by all mobile stations sharing that CDMA channel. Thus, there are strict requirements to be met by the base station amplifier, which is consequently one of the more expensive components of the base station.
For example, considering the signal intended to be transmitted in a particular 1.23 MHz CDMA channel, TIA/EIA standard IS-95A imposes strict limits on the amount of power which is allowed to exist in neighbouring CDMA channels. By limiting the amount of allowable "spillover", this effectively imposes a requirement for high linearity on the part of the base station power amplifier. It is to be understood that the expressions "in-band distortion" and "out-of-band distortion" hereinafter refer to distortion within the intended 1.23 MHz CDMA channel and outside the intended 1.23 MHz CDMA channel, respectively.
In a dynamic scenario, as more and more mobile stations are accommodated by the base station, the agglomeration of the individual spread data signals leads to a progressively more random waveform for the composite signal input to the power amplifier, effectively taking on a Gaussian probability density function (pdf) centered about zero volts. In order to keep the out-of-band distortion to a minimum, extreme voltage values, although rare due to the Gaussian pdf, must be guaranteed to fall within the operating range of the power amplifier. That is to say, the dynamic range required of the power amplifier is related to the maximum voltage swing of the composite spread-spectrum waveform.
Unfortunately, the cost of a power amplifier increases dramatically with the input voltage range over which it is required to operate. Therefore, it is of interest to keep the input voltage range relatively low, i.e., to limit the range of the composite waveform created by the multiple spread data signals. Clearly, other than employing a more costly power amplifier with a higher input voltage rating, one solution for limiting the range of the composite waveform is to correspondingly limit the number of users sharing the 1.23 MHz CDMA channel.
However, if there is a high density of mobile stations in a particular area, then the maximum permitted number of users in a given CDMA channel may easily be exceeded, requiring the use of additional CDMA channels or even base stations, which is a costly option for the telecommunications service provider. This and other prior art solutions are evidently inefficient, since they lead to a substantial investment for handling the occasional, although natural occurrence of high peak voltages as the number of users increases.